1. Field of the Invention
The present invention relates to a battery using electrodes at least one of which is formed from a three-dimensional porous body such as a foamed metal, a nonwoven cloth metal, or the like, having an active material carried therein.
2. Description of the Related Art
In order to improve an active material filling density to increase a battery capacity, some electrodes use foamed metal or non-woven cloth-like metal. For example, a nickel-hydrogen secondary battery uses an positive electrode made of a foamed nickel (foamed metal), nickel fiber felt (non-woven metal) or the like. The foamed nickel may be created by plating urethane foam containing carbon to have conductivity with nickel and baking it to blow off the compositions of urethane and carbon to leave only the foamed metallic nickel. Such a foamed nickel constitutes a very porous three-dimensional body with a large number of nickel frames three-dimensionally coupled with one another in a network (mesh). The nickel fiber felt may be created by baking nickel slender fibers made by e.g. chattering vibration into a felt (nonwoven) cloth. Such a nickel fiber felt also constitutes a very porous three-dimensional body with a large number of nickel fibers three-dimensionally coupled with one another in a network (mesh). Therefore, if powder (non-aqueous) of nickel hydroxide which serves as an active material dispersed in a dispersant such as water is applied in these three-dimensional porous body and dried, a large quantity of active material can be surely carried in the gaps among a large number of frames or fibers coupled in a network to increase the filling density of the active material. This increases the battery capacity greatly.
Meanwhile, the three-dimensional porous body must collect current through a current collector of a metallic sheet for its connection to a terminal of a battery. However, it is difficult to connect the current collector to the three-dimensional porous body containing the active material directly by welding, because the active material hinders welding and the mechanical strength of the frames or fibers of the three-dimensional porous body is low.
Therefore, conventionally, a part of the three-dimensional porous body was previously pressed and was caused to carry the active material. The current collector was welded or press-fitted to the pressed area. Specifically, if the three-dimensional porous body is pressed, the frames of fibers of the pressed area is formed as a metallic sheet so that the active material is substantially not applied to the pressed area. For this reason, after the active material is carried on the three-dimensional porous body, the current collector can be welded to the pressed area, thus assuring the connection. In some cases, after the active material was once carried on the three-dimensional porous body, ultrasonic vibration is applied to an area of the three-dimensional porous body to remove the active material, and the current collector was welded to the removed area.
In some cases, belt-shaped steel which serves as a current collector was crimped on the entire surface of the three-dimensional porous body.
However, conventionally, as described above, with an area of the three-dimensional porous body pressed previously or the active material removed, the current collector must be welded or crimped on the three-dimensional body. In addition, since such a step was required for each of the electrodes, the productivity of the battery was attenuated.
Further, application of slight pressing force to the three-dimensional porous body crushes the frames or intimately couples the fibers with each other, and application of slight pulling force thereto extends the network portion to reduce the thickness of the entire three-dimensional porous body. This makes it impossible to carry the active material on the three-dimensional body. In addition, increasing the pulling force cleaves the three-dimensional body easily. Particularly, where the current collector is welded or crimped on an area of the three-dimensional porous body, when force is applied during the operation of assembling the battery or the vibration at the time of using the battery, the three-dimensional porous body is easily cleaved or cut at its welding part or crimping part. Moreover, when such a current collector is welded or crimped to a portion of the three-dimensional porous body, the electric resistance of the portion which separates from the current collector is increased.